RD100 Bias runaway voltage

Hi all,

I have a problem with my RD100 built June 1981. The circuit is the GD-2A, with 6L6 output tubes. The amp gets a horrible “hiss” and terrible sound when the gain goes above 3.  There is a noticeable “bump” in the sound and then the terrible hiss. The amp was always hissy when the gain was turned up, but this is far worse.

I thought the problem might be the Driver Transistor Bias Calibration because the amp had sat unused for quite a while.  Also, the amp had blown its fuse… which had never occurred before.

Following the instructions in Service Bulletin #1, I checked the voltages on the 3.9 ohm emitter resistors, and brought them as close to spec as possible.  One side at 25mV and the other read at 30mV which the Bulletin said would be OK.

I noticed, however, that over a number of minutes the voltages slowly increased. After more than an hour, the lower side was over 50mV and the higher side was over 70mV.  Yikes!

Any thoughts?